In young adult women compact bone mass is relatively stable up to the menopause, but trabecular bone loss occurs at an annual rate of 0.6%. This is accompanied by a negative calcium balance of -10 to -20mg/day. This is not as large as that occurring after the menopause (-40mg/day) when loss of compact bone adds -30mg/day to the negative balance. The documented low calcium intake (less than 800mg/day) in this population, as well as other nutritional factors, may influence the on-going bone loss. Axial bone mineral content (BMC) of the spine (dual-photon absorptiometry using 153Gd) and appendicular BMC of the radius (single-photon absorptiometry using 125I) will be monitored over a three-year period in series of 200 normal women aged 20-40 years; half of the group will be regular users of oral contraceptives and half will not. Spinal and radius BMC will be measured at annual intervals. Concomitant measurements will be made of 24-hour urinary calcium and hydroxyproline, serum chemistries and hormones (PTH, estrogens, vitamin-D metabolites). Nutritional histories and 24-hour dietary recalls will be obtained on 3 occasions for each subject. Physical activity will be assessed on 3 occasions by heart rate monitoring over a 24-hour period. The data will indicate the relation of premenopausal bone changes to calcium and vitamin-D status, and show the potential impact of nutrition, activity and oral contraceptive use on this loss. This study has direct implications for prevention of fractures in women later in life.